A Discrete Multi-Tone (DMT) communication system carries information from a transmitter to a receiver over a number of tones. The tones are also commonly referred to as sub-carriers or sub-channels. There are various sources of interference and noise in a DMT system that may corrupt the information signal on each tone as it travels through the communication channel and is decoded at the receiver. Because of this signal corruption, the transmitted data may be retrieved erroneously by the receiver. In order to ensure a reliable communication between transmitter and receiver, each tone may carry a limited number of data bits. The number of data bits or the amount of information that a tone carries may vary from tone to tone and depends on the relative power of the information and the corrupting signals on that particular tone.
A reliable communication system is typically defined as a system in which the probability of an erroneously detected data bit by the receiver is always less than a target value. The aggregate sources of corruption associated with each tone are commonly modeled as a single additive noise source with Gaussian distribution that is added to the information signal on that tone. Under these assumptions, the signal-to-noise power ratio (SNR) becomes a significant factor in determining the maximum number of data bits a tone can carry reliably.
The direct relationship between SNR and the bit rate is based on the key assumption of Gaussian distribution for noise. However, this assumption may not be completely valid in many practical situations. An important source of non-Gaussian impairment is, for example, phase noise. Phase noise may be a sampling-time phase mismatch between the transmitter and the receiver devices. This type of error may result from for example, phase jitter of the sampling oscillator on the transmitter side or poor phase lock on the receiver side. With such noise sources potentially present, a determination of the SNR may not accurately determine the reliable bit rate.